Pursuant to 35 USC xc2xa7119, this application claims the benefit of Japan Patent Application No. 2001-181871 filed Jun. 15, 2001.
The present invention relates to a heat-radiating electromagnetic wave absorber that is used in electronic products, such as information, imaging, and mobile communication equipments. More specifically, the present invention relates to a heat-radiating electromagnetic wave absorber that damps and absorbs electromagnetic noise generated by electronic components while dissipating heat produced by the devices.
Recently, digital electronic equipments and other electronic products employing the quasi-microwave band (100 MHz-3 GHz) and higher frequency ranges have come into wide use. Such products are mounted with electronic components in a high-density due to requirements of compactness and high performance. Such high-density mounted electronic products may have problems, such as electromagnetic wave hindrance or interference caused by the electromagnetic noise generated at the devices or decrease of properties due to heat. Therefore, it has been an important subject to deal with these problems.
In order to avoid the above problems, electromagnetic wave absorbers are used to damp and absorb the electromagnetic noise. Thermally conductive sheets (thermally conductive molded materials) are also used to effectively dissipate heat produced at the electronic components to the outside.
The recent high performance exothermic electronic components need countermeasures for both electromagnetic noise and heat at the same time. For such case, use of both materials, an electromagnetic noise absorber and an thermally conductive molded material, is cost consuming due to increased number of parts, and requires a larger mounting space.
To deal with above disadvantages, U.S. Pat. No. 6,284,363 proposes an electromagnetic wave absorbing thermally conductive silicone gel molded sheet, which absorbs electromagnetic waves and dissipates heat with a single member. This silicone gel sheet is formed from a silicone gel composition comprising metal oxide magnetic particles and a thermally conductive filler.
The silicone gel sheet proposed by above patent, however, uses Mnxe2x80x94Zn (Manganese-Zinc) ferrite or Nixe2x80x94Zn (Nickel-Zinc) ferrite, which have low thermal conductivity, as metal oxide magnetic particles. Thus the sheet has less thermal conductivity than existing thermally conductive sheets dispersed with only thermally conductive fillers, and does not have sufficient heat radiating properties.
Moreover, the current silicone gel sheet mentioned above has a disadvantage that the permeability of the sheet will decrease at high frequency band over 300 MHz, due to a restriction called xe2x80x9cSnoek""s Limitxe2x80x9d, since the sheet uses Spinel type of Mnxe2x80x94Zn or Nixe2x80x94Zn ferrites as metal oxide magnetic materials.
The following description describes the damping effect of electromagnetic noise for metal oxide magnetic materials based on Snoek""s equation given by equation (1).
fr(xcexcxe2x80x2xe2x88x921)=Is.xcex3/3xcfx80xcexc0xe2x80x83xe2x80x83(1) 
(fr: resonance frequency, xcexcxe2x80x2: real part of complex specific permeability, xcex3: gyromagnetic ratio, xcexc0: permeability in a vacuum, Is: saturation magnetization)
When a magnetic material is magnetized while increasing the frequency from a low frequency to a high frequency, the material generally causes domain wall displacement and rotational magnetization. The magnetic material becomes unable to follow the magnetic field change under certain frequency magnetism, then the material causes a magnetic resonance. The frequency at which this occurs is referred to as a resonance frequency fr When a permeability of the magnetic material depends on frequency, the complex specific permeability U can be represented as U=xcexcxe2x80x2xe2x88x92ixcexcxe2x80x3. This complex part xcexcxe2x80x3 expresses a delay of magnetization at a alternating magnetic field, and corresponds to a magnetic loss. As the magnetic material is magnetized while increasing the frequency, the real part xcexcxe2x80x2 of the complex specific permeability decreases, while the imaginary part xcexcxe2x80x3 of the permeability increases. The imaginary part xcexcxe2x80x3 becomes a maximum at the resonance frequency fr. Consequently, a possible frequency band range where the magnetic materials can be used as electromagnetic wave absorbers is in a band where the materials have a large magnetic loss, that is, in higher frequency band than the fr of the material where the xcexcxe2x80x3 is higher while the xcexcxe2x80x2 is lower (xcexcxe2x80x3 greater than xcexcxe2x80x2). On the other hand, the electromagnetic noise damping effect decreases in the range where xcexcxe2x80x3 is far from the fr in the higher frequency side, because the xcexcxe2x80x3 becomes too low in such ranges of high frequency.
The permeability of the Spinel type ferrite will now be discussed. Generally, the Spinel type ferrite has a small fr because of small saturation magnetization Is. Such small fr of Spinel type ferrite, lower than quasi-microwave band, results in the decreased permeability, causing the damping effect to be insufficient in the quasi-microwave band and higher frequency ranges.
Consequently, the current electromagnetic wave absorbing thermally conductive silicone gel sheet abovementioned does not have sufficient electromagnetic wave absorbing properties for the quasi-microwave band and higher frequency ranges, although the sheet has an improved electromagnetic wave absorbing properties for the range of 30 to 300 MHz. In other words, the current silicone gel sheet is not suitable to be used for the quasi-microwave band and higher frequency ranges.
The present invention aims to address the above disadvantages and its object is to provide a heat radiating electromagnetic wave absorber that has excellent electromagnetic wave absorbing properties in quasi-microwave and higher frequency ranges as well as thermal conductivity.
The heat-radiating electromagnetic wave absorber is provided. The absorber is molded in a predetermined form from a mixed composition comprising soft magnetic powder and a thermally conductive filler in an organic matrix, wherein the soft magnetic powder includes Fexe2x80x94Si (iron-silicon) alloy powder.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.